Vacuum-tube-echo suppressor



. 1 Nov 1927 G. CRISSON VACUUM TUBE ECHO SUPPRESSOR Filed Sept. 18, 1925 INVENTOR G. Cr/wsm z/ BY Patented ii, 3%27.

NEW JERSEY, ASSIGNDR T0 AMERICAN TELE- PANY, A CORPORATION OF NEW YORK.

VACUUM-TUBE-ECHO SUPPRESSOR.

Application filed September 18, 1925. Serial No. 57,225. 1

This invention relates to vacuum tube relays and more particularly to transmission circuits in which vacuum tube relays are employed for the suppress When a subscriber talks sion circuit, echo currents back in the reverse ion of echoes.

over a transmisare transmitted direction due to reflec tion either at the terminal points or at points where balancing arrangen'ients are provided in connection with repeaters.

may be prevented trom arrivecho currents The ing at the originating subscribers set by employing echo suppressors at points where the transmission circuit involves individual paths for transmitting in each di rection. An echo suppressor unit is associated witheach path and when transmission occurs over one unit associated with the suppressor path that path operates to disable the path transmitting in the reverse direction.

Echo suppressor units as heretofore constructed have usually com fier detecting arrangement pedance input prised an ampliwith a high imwhich is bridged across one of the transmitting'paths and operates in response to voice. currents'or other signaling currents to produce rectified currents to operate a relay. The relay is a mechanical relay arrangement whose contacts are arranged to disable the opposite transmitting path when the rectified current tlows relay winding.

While it has also been pressors,

through the proposed to use as echo supbeen impractical for a number of reasons. The disabling operation resulted in clicksin the talking circuit. Furthermore the hangover time could not be long and both the operating periods were dependent on voice currents. between noise currents and of the operating no discrimination made sufficiently and hangover the magnitude Moreover voice currents was possible in the operation of the circuit. Thereiore present invention is to provide a an object of the voice operated echo suppressor employing vacuum tube relays which will operate quickly and without clicks to disable the path of not seine used, which wii transmission ni have a tie in e and controllable hangover interval substantially independent of the magnitude of the voice currents operating the device, and which will so discriminate against noise currents as to permit its action to be very sensitive to ,voice currents greater in magnitude than the noise currents.

Other and further objects of the invention will be clear from the following detailed description when read in connection with the accompanying drawing, Figures 1 and 2 of which are circuit diagrams illustrating two embodiments of the invention and Fig. 3 of which is a series of curves illustrating the operation of the circuit.

Referring to Fig. 1, L and L designate two transmission paths including amplifiers A and A These paths may be the two sides of a four-wire circuit or they may be the two transmission paths of a 22 repeater. An echo suppressor combination ES is bridged across the path L on the output side of the repeater A and operates in response to voice currents transmitted over the path L to practically short-circuit the output side of the amplifier A of the path L A similar echo suppressor combination BS is bridged across the path L to disable the path L This echosuppressor combination is merely indicated symbolically and will be understood to be identical in all respects with the combination E3 which is illust *ated in detail.

The relay member of the echo suppressor combination E comprises two vacuum tubes 1 and 1 which are connected in pushpull relation. The two plates are connected to the ends and the two filaments to the middle of one winding of a transformer let, whose remaining winding is bridged across the line L as indicated. The grids of the two tubes are connected to each other and between the grids and the filament is placed an arrangement for varying the voltage normally impressed upon the grids so that the impedance of the combination which bridged across the circuit L can. controlled. I

Without for the moment. goin l detailsor" the arrangeinent t'or var tween the filaments and plates of the tubes so that the impedance of the plate circuits of the tubes will be relatively low. The minimum impedance will, of course, depend upon the character of the tubes employed but for the case of an ordinary vacuum tube such as is used for telephone repeater purposes, it

might be in the neighborhood of 2,000 ohms. As an impedance of 2,000 ohms will not, in general, constitute a sufficient shunt across the circuit L to disable it, this impedance may be stepped down to a very low value by means of a stepdown transformer 14. Consequently, when the potential of the grids is so changed as to permit a maximum current to flow in the output circuits of the tubes, the line L will, in elfect, be shunted by an impedance which constitutes practically a short circuit, although it will be understood that this impedance may be given a somewhat higher value if desired.

The arrangement for controlling the potential upon the grids of the tubes 1 and 1 comprises a battery 2 connected between the filamentsof the tubes 1 and 1 and the grids thereof through a resistance 3 so that normally the grids have a high negative potential with respect to the filament and no appreciable current is shunted away from the line L when speech currents pass over it. In order tovary the potential applied to the grid when transmission takes place over the line L a vacuum tube amplifier 4 is arranged with its input circuit bridged across the line L. This amplifier is shown as comprising a single vacuum tube, but obviously any combination of tubes or any other type of amplifying device might be used to secure the desired result. The secondary winding of the output transformer of Y the amplifier tube comprises two parts 5 and 6 which are connected through rectifiers 7 and 8 and a resistance 10 to one terminal of a condenser 11 which is connected in parallel with the resistance 3 of the grid filament circuitof the vacuum tubes 1 and 1. The common pointof the windings 5 and 6 is connected to t 1e opposite terminal of the condenser 11 through a battery 9, 'wilOSe purpose will be described later. The rectifiers 7 and 8 may be of any well known type but are shown as two-element vacuum tubes.

hen an alternating voltage is impressed upon the amplifier 4 it sets up electromotive forces in the windings 5 and 6 which cause a direct current to flow through the rectifiers to charge the condenser 11 through the resistance 10. The windings 5 and 6 are so connected that the half waves of one polarity are sent through rectifier 7 while the other half waves are sent from the winding 6 through the rectifier 8, but both tend to charge the condenser 11 in the same direction. The use of two windings and two rectifiers is not essential to the operation of the system but tends to make the charging of the condenser more rapid, thereby increasing the speed of operation of the system. The rectified potential is applied to the condenser 11 in such a direction as to make the potential of the grids less negative with respect to the filament so that the apparatus comprising the tubes 1 and 1 operates as previously described to shunt the line L and thus reduce and substantially extinguish transmission over this line. In other words, the windings 5 and 6 and the rectifiers 7 and 8 are so connected that the potential applied across the terminals of the condenser 11 tends to oppose the normal negative potential due to the battery 2.

It is desirable and is, in fact, one of the objects of the invention that the shunt be placed upon the line L as nearly instantaneously as possible. It therefore becomes desirable to arrange the condenser 11 so that it will be charged as quickly as possible. If the resistance 10 were not present the condenser 11 would be instantaneously charged to the full rectified voltage and hence the operation of the echo suppressor to apply the shunt to the line L would be instantaneous. It is necessary to provide the resist-- ance 10, however, in order to enable the use of the rectifier 13 and battery 12, which serve to limit the potential to which the condenser 11 may be charged, as will be described later. The presence of the resistance 10 prevents the condenser 11 from attaining its full charge immediately, although the time required for the condenser to attain the necessary charge for satisfactory operation may be cut down in a manner to, be described later.

The resistance 3 is made large so that the charge in the condenser 11 will be retained for a sufficient time after the transmission over the line L has ceased, to take care of the fact that it requires a definite time for the last wave of voice current from the line L to pass back over the line L as an echo and arrive at the point where the shunt is applied through the transformer 14. This socalled hangover interval is required in all echo suppressor systems. The speed with which the condenser 11 discharges, and consequently the effective length of this hangover interval, can be adjusted by changing the magnitude of the resistance 3. With lltl ' L and shorter when given values of resistance 3 and with a given capacity of the condenser 11 the time required for the condenser to discharge and hence for t he system to berestored to the free transmitting condition depends upon the charge in the condenser 11. In other words, the hangover interval will be long when loud volumes of speech are passing over the line the volume of speech is low. The reason that this is so will be explained later but it is this fact that necessitates the provision for the rectifier 13 and battery 12 to limit the potential to which the condenser 11 is charged.

It will be noted that the rectifier 13 and the batteries 12 and 2 in series are connected in parallel with the condenser 11. The batteries 2 and 12 are series-aiding and are so poled with respect to the-rectifier 13 that they oppose current flowing through this rectifier'from the rest of the circuit. On the other hand, the rectifier 13 is so connected with respect to the circuit including the windings 5 and 6 and the rectifiers 7 and 8 that the rectified potential which charges the condenser 11 is in such a'direction as to tend to cause a current to flow through the rectifier 13. Consequently, as soon as the rectified potential is raised high enough to overcome the opposing potential of the batteries 2 and 12 in series, current flows through the rectifier 13 and through the batteries 12 and 2 in series, thereby shunting the condenser 11 and preventing the potential applied to its terminals from rising appreciably higher. The combined potential of the batteries 2 and 12 is so set that the rectifier 13 will come into play at a otential somewhat greater than that which is necessary to cause the proper operation of the tubes 1 and 1' to shunt the line L,. It will be obvious, of course, that if 'the resistance '10 were not present the rectifier 13 could not exercise any shunting effect on the condenser 11, but the fullirectified potential would be applied to said condenser in any case.

It will be recalled that the resistance 3 was provided to regulate the hangover time required for the capacity 11 to discharge, and that the resistance 10 was provided to enable the by-pass circuit through the rectifier 13 to limit the potential applied to the condenser 11. It is, of course, desirable that the condenser 11 be charged to the limited value as quickly as possible and that the time of discharge shall be'independent of the rectified potential supplied by rectifiers 7 and 8 so that the hangover time will always be the same. In order to understand how the condenser 11 and its associated apparatus accomplish this result, reference'may be had to the curves of Fig. 3.

In this figure, let E represent the potential which must be applied across the terminals of the condenser 11 in order to bring the potential of the grids of the tubes 1 and 1 to the desired value to disable the circuit L,. If, now, a rectified potential E, is applied to the circuit leading to the terminals of the condenser 11 and this circuit includes no resistance, it will be obvious that the condenser would be fully charged to the potential E instantly. If, however, resistance 10 is included in series with the condenser and a rectified potential E is applied to the circuit, it will take a definite time to build up to approximately the full potential E, across the terminals of the condenser; Now, the time required to build up to substantially the final voltage across the terminals of the condenser will be the same regardless of the magnitude of this voltage, but the time to build up the charge to some definite lesser value such as E will vary with the applied voltage the time required becoming smaller as the voltage becomes greater. This is illustrated by the curves a and b of Fig. 3 which show the building up of the potential difi'erence between the terminals of the condenser 11 for two difierent values of applied voltage, E and E,. These curves both approximately reach their asymptotes E and E in the same length of time, but curve a crosses the line corresponding to E in much less time than curve I). It is therefore apparent that in the case of the higher applied potential the charge acroSs the condenser is built up to E much more quickly than in the case of the lower potential represented by curve b. With a high resistance 10 in the circuit, the amplifier 4 should amplit'y the voice currents suificiently to pro-- duce a much higher rectified potential than is actually necessary to produce the desired charge across the condenser 11. a

Let us now suppose that the potential to which the condenser is charged is limited so that it does not rise above the value E If, when the applied potential ceases, the condenser were short-circuited the potential would instantly'drop from E to zero. As already stated, however, it is desirable to have a definite hangover time; consequently the resistance 3 is provided and it may be adjusted to give any desired hangover time. The curves d, e and 7 represent the manner in which the condenser discharges with different values of the resistance 3. The time required for the charge to drop to value E (at which it may be assumed that the relay tubes 1 and 1' no longer disable the line L,) will be much less when the resistance 3 is small, as represented by the curve f, than when the resistance is large, as represented v The resistance 3 alone deby the curvee. termines the time required to discharge the condenser to a given minimum value as there is no discharge path through the rectifier 13 due to the opposing voltage ofthe batteries 2 and 12, or through the rectifiers &

7 and 8 owing to the fact that these rectifiers are non-conductive for the direction in which the discharge would take place.

The desirability of preventing the condenser 11 from being charged above a predetermined maximum limit such as E will be evident from a comparison of the curves (Z and g. The curve d represents the discharge of the condenser from a. value E for a given adjustment of the resistance 3. Similarly, the curve 9 represents the discharge of the condenser from a value E with the same resistance. Now, obviously, the time required for the condenser to discharge to the value E in the two cases is markedly dillerent. Since it is necessary to apply a large rectified potential in order to produce a quick charge of the condenser to the operating value, the desirability of preventing the condenser from charging to a value materially greater than the operating value becomes at once apparent, because satisfactory behavior of the suppressor requires that the hangover interval be substantially constant.

A further feature of the circuit of Fig. 1 resides in the provision of the battery 9, which is for the purpose of making the system discriminate against noise and enable a larger gain to be used in the amplifier 4. The battery 9 is so poled that its electromotive force is opposed to the current which will pass through the rectifiers7 and 8. Consequently, before any current can pass through the rectifiers the applied potential must be greater than the electromotive force of the battery 9 and, of course, it must be in the opposite direction. The battery 9 is so adjusted that the normal noise on the line L will be insufficient to pass current through the rectifier-s 7 and 8 and consequentl' the noise will produce no charge in the condenser 11'. When, however, the speech volume becomes great enough to override the noise, condenser 11 is charged and the transmission over the line L is interrupted. Since the influence of the line noise is counteracted by the electromotive force of the battery 9, the gain of the amplifier 4 may be made as great as necessary in order to produce the prompt charging of the condenser 11 and the proper operation of the vacuum tubes 1 and 1. Of course, if the gain of the amplifier 4 is increased the voltage of the battery 9 must also be raised to offset the larger effect of noise but the excess of speech over wire will also be increased.

A modified arrangement is shown in Fig. 2 in which the vacuum tube relay operates to disable the echo path, but the hangover time is determined by a mechanical relay. The vacuum tube relay comprises vacuum tubes 1 and 1' as before, the operation of these tubes being controlled by building up a charge across the terminals of the condenser 11. The circuit for charging the condenser includes .the rectifiers 7' and 8 and transformer windings 5 and 6, just as in the case of Fig. 1, but in the present instance the limiting arrangement comprising the rectifier 13 and the battery 12 is not necessary since the resistance 3 will be made suficient ly low to cause a quick discharge of the condenser, and the hangover time will be determined by the slow-release relay 21, to be described later. As no limiting device is required, the resistance 10 may be omitted and consequently when voice currents are applied to the input of the amplifier 4 the rectified potential produced by the rectifiers 7 and 8 charges the condenser 11 rapidly, with the result that the relay comprising the tubes 1 and 1 promptly disables the line L 1 An additional winding 15 is provided for the transformer inthe output circuit of the amplifier 4 and a rectifier tube 16 is connected to this additional winding so that the amplified voice currents from the amplifier 4 will be rectified and the direct current component of the rectified current will be passed through the winding of a relay 17. A condenser 18 is bridged circuit of the rectifier 16 to by-pass the alternating component of the rectified current. The relay 17 closes, by means of its contact 19, a local circuit including a battery 20 and the slow-release relay 21. The relay 21 does not pull up its armature instantly, but when it does pull up its armature, closes contact 22 to short circuit or otherwise disable the line L independently of the shunt through the transformer controlled by the vacuum tube relay. An instant later the contact 23 opens and removes the rectified charging current from the condenser 11. The condenser then discharges through the resistance 3 and removes the effective shunt through the transformer 14. The line L is now held disabled at contact 22 as long as the voice currents are applied through the amplifier 4 to the rectifier 16. WVhen the voice currents cease the relay 21 releases after a hangover interval determined-by its time of release and the short circuit is then removed from the line L It will be noted that here the vacuum tube relay arrangement operates to disable the line L rapidly. After an interval the line is held in a disabled condition by the mechanical relay 21 and the vacuum tube relay is released, the disabling of the line being thereafter under the control of the mechanical relay. The hangover time being determined by the slow-release relay will always be the same for a particularladjustment of said relay and will not be determined by the potential required to charge the condenser 11.

across the output many other plate circuit pedance It will be obvious that the general principles herein disclosed may be embodied in forms and arrangements without departing from the spirt and scope 0 the appended claims.

What is claimed is:

1. In a transmission system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having its plate circuit paths, means to apply a normal potential to the grid of said tube to maintain the impedance bridged across said path large, and means responsive to signaling currents transmitted over the other path to change the grid. potential to reduce the output impedance of said tube, thereby disabling said first mentioned path.

2. In a transmission system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having its bridged across one of said paths, means. to apply a normal potential to the grid of said tube to maintain the imbridged across said path large, means responsive to signaling currents transmitted over the other path to produce a potential determined by said signaling currents, and means to apply said potential to the grid of said tube in such a direction as to reduce the output impedance, thereby disabling said first mentioned path.

3. In a transmission system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having'its plate circuit bridged across one of said paths, means to apply a normal potential to the grid of said tube to maintain the impedance bridged across said path large, means to apply a rectified potential corresponding to signaling currents from the other transmission path to the grid of said Vacuum tube in such a direction as to reduce the output impedance, thereby disabling said first mentioned path.

4. In a transmission'system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having its plate circuit bridged across one of said paths, means to apply a normal potential to the grid of said tube to maintain the impedance bridged acrosssaid path large, a storing device associated with said grid circuit, and means to apply a rectifiedpotential corresponding to signaling currents from the other path to charge said storing device in such a direction as to reduce the'output impedance of the tube and disable said first mentioned path.

5. In a transmission system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having its plate circuit bridged across one of said paths, means to apply a normal potential to the grid of said tube to maintain the impedance bridged across one of said bridged across said path large, a storing device associated with said grid circuit, means to apply rectified potential corresponding to signaling currents from the other path to charge said storing device in such a direction as to reduce the output impedance of the tube and disable said first mentioned path, and means to limit the rectified potential applied to said storing device. r

6. In a transmission system, two one-way "transmission paths for transmitting in opposite directions, a vacuum tube having its plate circuit bridged across one of said paths, means to apply a normal potential to the grid of said tube to maintain the impedance bridged acrosssaid path large, a storing device associated with said grid circuit, means to apply rectified potentials corresponding to signaling currents from the other path to charge said storing device in such a direction as to reducethe output impedance of the tube and disable said first mentioned path, and means to prevent the application of rectified potential to said storing device in response to Weak currents of the order 0% noise currents transmitted over said second transmission path.

7. A transmission system comprising a pair of transmission paths for transmitting in opposite directions, a disabling relay associated with one of said paths comprising a pair of vacuum tubes connected in a pushpull circuit with the plate circuits of said tubes bridged across said path, means to apply a normal potential to the grids of said tubes such that the impedance of the plate circuit will be substantially infinite, and means responsive to signaling currents transmitted over the other path to change the potential of said grids to reduce the output impedance, thereby disabling said first mentioned path. v

8. A transmission system comprising a pair of transmission paths for transmitting in opposite directions, a disabling relay as sociated with one of said paths comprising a pair of vacuum tubes connected in a pushpull circuit with the plate circuit of said tubes bridged across said path, means to apply a normal potential to the grids of said tubes such that the impedance of the plate circuit will be substantially infinite, means Cir pull circuit with the plate circuits oi said tubes bridged across said path, means to apply a normal potential to the grids of said. tubessuch that the impedance of the output circuit will be substantially infinite, means to apply a rectified potential corresponding to signaling current from the other transmission path to the grids of said tubes in such a direction as to reduce the output impedance, thereby disabling said first mentioued path. v

10. A transmission system comprising a pair of transmission paths for transmitting in opposite directions, a disabling relay associated with one of said paths comprising a pair of vacuum tubes connected in a push pull circuit with the plate circuits of said tubes bridged across the said path, means to apply a no: nal potential to the grids of said tubes such that the impedance of the plate circuit will be sucstantially infinite, storing device included in the common path of the grids oi said tubes, and means to apply rectified potential corresponding to signaling currents from the other path to charge said storing device in such a direc tion as to reduce the output impedance of the tubes and disable said first mentioned path.

11. Atransmission system comprisin pair of transmission paths for transmitting in opposite directions, a disablingrelay associated withone of said paths comprising a pair of vacuum tubes connected in a pushpull circuit with the plate circuits of said tubes bridged across said path, means to apply a normal potential to the grids of said tubes such that the impedance of the plate circuit will be substantially infinite, a storing device included in. the common path of the grids of said tubes, means to apply rectiiied potential corresponding to signaling currents from the other path to charge said storingdevice in such a direction as to reduce the plate impedance of the tubes and disable said first mentioned path, and means to limit the rectified potential applied to said storing device.

12. A transmission system comprising a pair of transmission paths for transmitting in opposite directions, a disabling relay associated with one of said paths comprising a pair of vacuum tubes connected in a push-pull circuit with the plate circuit of said tubes bridged across the said. path, means to apply a normal potential to the grids of said tubes such that the impedance ofthe plate circuit will be substantially infinite, a storin device included in the common path of the grids of said tubes, means to apply rectified potential corresponding to signaling currents from the other path to charge said storing device in such a direction as to reduce the plate circuit impedance of the tubes and disable said first mentioned e rners path, and means to prevent the application of rectified potential to said storing device in response to Weak currents of the order of noise currents transmitted over said second transmission path.

13. in a transmission system, two one-way transmission paths for transmitting in opposite directions, a three element vacuum tube having its circuit so associated with one of said paths as to disable said path without producing clicks therein when said tube is operated, means to normally apply a potential to the controlling electrode of said tube to prevent said tube from disabling said path, and means responsive to signaling currents transmitted over said other path to cause said tube to quickly disable said iirst path.

let. in a transmission system, two oneway transmission paths for transmitting in opposite directions, a three element vacuum tube having its circuit so associated with one of said paths as to disable said path without producing clicks therein when said tube is operated, means to normally apply a potential to the controlling electrode of said tube to prevent said tube from disabling said path, means responsive to signaling currents transmitted over said other path to cause said tube to disable said first path, and means to maintain said first path disabled for a predetermined time after said signaling currents have ceased.

15. In transmission system, two one-way transmission paths for transmitting in opposits directions, a vacuum tube having its circuit'so associated with one of said paths disable said path without producing clicks therein when said tube operated, iii-cans to normally prevent said tube from disabling said path, means responsive to signaling currents transmitted over said other path to cause said tube to disable said first path, and means to maintain said first path disabled after said signaling currents have ceased for a predetermined time which is substantially independent of the strength of said signaling currents.

16. In a transmission system, two one-way transmission paths for transmitting in opposite directions, a vacuum tube having its circuit so associated with one of said paths as to disable said path without producing clicks therein when said tube is operated, means to normally prevent said tube from disabling said path, means responsive to signaling currents transmitted over said other path to cause said tube to disable said first path, and means to prevent said tube from responding to weak currents of the order of noise currr is.

In testimo). whereof, I have signed my name to this s ecification this 17th day of September 1925.

GEORGE CRISSON. 

